Office 365 is always a keep-paying-forever "subscription" license.
Before 2013, all Office was one-time-payment / "perpetual" license.
Since 2013, all Office with year in name: Office 2013 ..., Office 2016 ... , Office 2019 ... , are one-time payment.
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365 gives you the option of installing on many OS's: Windows, Mac, iPad, Android
one-time payment licences always are OS specific, ie PC or Mac, cannot switch
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Creating a 64 bit version requires extra time / MONEY! So if MS did not create it originally, they will not NOW go back and create it for an old version that is not making them any money. ie 2011 is less than a year from it's ultimate end of life ... ,
no money in it, no chance they will make 64 bit.
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On Windows side, 2010 was first 64 bit version
On Mac side, 2016 is first mention of 64 bit I can find
https://docs.microsoft.com/en-us/office/troubleshoot/office-for-mac/office-2016-for-mac-64-bit-upgrade
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The above documentation has following statement:
<snip>
64-bit only
The Mac operating system has been 64-bit for several years. Unlike for Windows, there's no choice between running a 32-bit or 64-bit version of the Mac operating system. Similarly, we won't offer a choice between 32-bit and 64-bit versions of Office 2016
for Mac. After each "channel" is transitioned per the rollout schedule, only 64-bit builds will be available.
</snip>
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When you buy a license, subscription OR one-time-payment, you have the right to install either bit version you need.
On Windows side, I have seen documentation MS has started defaulting installations to 64 bit